"After the end of the Cold War, NATO reduced its capabilities at sea, especially in the fight against submarines. We have practiced less and lost skills," Stoltenberg told the German news outlet Frankfurter Allgemeine Sonntagszeitung.
At the same time, the secretary-general said that Russia was increasing its capabilities in this sector.
"Russia has invested heavily in its navy, especially in submarines. Since 2014, 13 more submarines have been commissioned. Russia's submarine activities are now at the highest level since the Cold War," he said.
In early November, Stoltenberg said NATO members had agreed on instituting a new adaptive command structure to improve the alliance’s ability to move troops across Europe and to create a cyber operations center to adapt to the changing security environment.